Re: recursive spinlocks. Shoot.

[Jan Hudec <bulb@ucw.cz>]

On Sun, May 18, 2003 at 09:09:54PM +0200, Peter T. Breuer wrote:
> In article <20030518182010$0541@gated-at.bofh.it> you wrote:
> > On Sun, 18 May 2003, Peter T. Breuer wrote:
> >> Here's a before-breakfast implementation of a recursive spinlock. That
> 
> > A looong time ago I gave to someone a recursive spinlock implementation
> > that they integrated in the USB code. I don't see it in the latest
> > kernels, so I have to guess that they found a better solution to do their
> > things. I'm biased to say that it must not be necessary to have the thing
> > if you structure your code correctly.
> 
> Well, you can get rid of anything that way. The question is if the
> interface is an appropriate one to use or not - i.e. if it makes for
> better code in general, or if it make errors of programming less
> likely.

I dare to disagree. It makes for more messy code in general and might
result in the obvious bugs to be replaced by subtle ones that are far
harder to debug.

> I would argue that the latter is undoubtedly true - merely that
> userspace flock/fcntl works that way would argue for it, but there
> are a couple of other reasons too. 

No ;-) Only fcntl does.

> Going against is the point that it may be slower.  Can you dig out your
> implementation and show me it?  I wasn't going for assembler in my hasty

It's also a waste of memory. There are many structures that have a lock
per instance and four extra bytes (for the owner) would be noticeable.
Not that memory is so precious resource, but cachelines may be.

> example.  I just wanted to establish that it's easy, so that it becomes
> known that its easy, and folks therefore aren't afraid of it.  That both
> you and I have had to write it implies that it's not obvious code to
> everyone.

It's not about weather it's easy. It's about weather it's useful.

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						 Jan 'Bulb' Hudec <bulb@ucw.cz>